1. Field of the Invention
The present invention relates to a so-called holey fiber comprising a solid core region and a cladding region in which plural holes extending along the core region are provided.
2. Related Background Art
A so-called holey fiber comprises a solid core region extending along a longitudinal direction of the fiber and a cladding region in which plural holes, extending along the core region while surrounding the core region, are provided. FIG. 1 is a view illustrating the cross-sectional configuration of a conventional holey fiber 10. There is illustrated, in FIG. 1, the arrangement of the plural holes 13 in a cross section orthogonal to the longitudinal direction of the holey fiber 10. In FIG. 1, circles illustrated by a solid line in the cladding region 12 represent holes 13. On the other hand, circles illustrated by a dot line do not represent holes, but represent regions or positions (the same applies to FIGS. 2 and 3 which will be described later).
As illustrated in FIG. 1, the holey fiber 10 comprises a solid core region 11 existing at the center portion of the fiber 10 and extending along the longitudinal direction of the fiber 10 (the direction perpendicular to the paper face) and a cladding region 12 surrounding the core region 11. In the cladding region 12, there are provided plural holes 13 having a round-shaped cross-section and extending along the longitudinal direction of the fiber 10. The portion of the cladding region 12 other than the holes 13 and the core region 11 are comprised of silica glass and may occasionally contain secondary constituents or dopants to be added. In the cross-section orthogonal to the longitudinal direction, the plural holes 13 in the cladding region 22 are arranged so as to constitute plural layers (four layers in FIG. 1) about the core region 11. More specifically, the holes 13 constituting the respective layers are arranged so as to have six-fold rotational symmetry about the core region 11.
The six holes 13 constituting the first layer (the innermost layer) closest to the core region 11 are provided at six vertexes of a regular hexagon. The twelve holes 13 constituting the second layer (the second layer from the core region 11) are placed at the six vertexes of a regular hexagon and the middle points of the respective sides of the regular hexagon. The eighteen holes 13 constituting the third layer (the third layer from the core region 11) are placed at the six vertexes of a regular hexagon and the points trisecting the respective sides of the regular hexagon. The twenty-four holes 13 constituting the fourth layer (the forth layer from the core region 21) are placed at the six vertexes of a regular hexagon and the points quadrisect the respective sides of the regular hexagon. The regular hexagons of the respective layers have the same center position and the same orientation. The pitch L between adjacent holes 13 from among the holes 13 constituting the respective layers (the distance between the center positions of adjacent holes 13) is uniform. Further, the diameters d of the total of sixty holes 13 are generally uniform.
By properly designing the pitch L of the arranged holes 13, the diameter d and the like, the holey fiber 10 having the aforementioned configuration enables a greater absolute chromatic-dispersion value than those of standard optical fibers having an entirely-solid configuration and enables larger nonlinearity than those of conventional highly-nonlinear fibers. Further, it is possible to reduce the bending loss. Therefore, researches and developments have been conducted about holey fibers utilizing the characteristics as stated above.
For example, in a holey fiber described in Document 1 (K. P. Hansen, et al., “Fully Dispersion Controlled Triangular-Core Nonlinear Photonic Crystal Fiber”, 0FC2003, PD2), a apart of the holes in the first layer closest to the core region are omitted. This holey fiber enables substantially zero chromatic dispersion, a small absolute dispersion slope and large nonlinearity, near a wavelength of 1.55 μm which is generally used as a signal light wavelength region in optical communication. By employing the holey fiber described in Document 1, it is possible to develop a nonlinear optical phenomenon, with a high efficiency, near a wavelength of 1.55 μm, whereby it is to be expected to realize various types of applications.